If two people share Esperanto as a common language and neither knows the native language of the other, then the scenario you are describing would require them both to translate everything on the fly into Esperanto. Having the game presented in Esperanto permits both players to easily use the same terminology to collaborate during cooperative play.
I assume that you don't speak Esperanto, so it makes sense that you would say that. I don't speak Chinese, Russian, or French, so I don't really care if the game is available in those languages, BUT I can see how it would be useful to others, even though many games are only available in English/Spanish.
It is not difficult to add a new language to the game, so it is not necessary to limit consideration only to languages known by many people. The flaw in that thinking is that there is also no way to guess how many German, Spanish or Chinese speakers (who don't know English) will be interested in purchasing the game. One could argue that the most efficient approach would be to only publish the game in English and Spanish like many games do. But additional languages are added because variety is the spice of life.
There aren't many things in the game that can't be simply translated as you speak if you know two languages. The things that might not translate perfectly are POI names and perhaps perks. These don't really even need perfect translations, though. It isn't like POI names are real world locations you need to get correct. You can call them anything you want. Other things are easily translated as you speak. "Can you make me a tier 4 stone axe?" would translate easily between languages, not needing any in-game translation. You do have a point in regards to things like POI names but these aren't regularly needed to be said, so the need to translate is minimal, imo.
There is a difference between saying a language that isn't a native language is important versus saying a native language is important. If someone speaks only their native language, they can't easily play a game that isn't in that language. On the other hand, if someone speaks their native language and an auxiliary language, if the game is in their native language, they can easily play it. Two very different things. As far as knowing sales numbers, I'm sure they have a good idea how many people buy the game in various countries and can estimate quite well which languages are needed to get the most people playing the game.
Let me ask you this... You obviously speak English. I'm assuming that isn't your native language? I would expect that many people who decide to learn an auxiliary language will also learn one of the top languages spoken in the world as well (or instead of an auxiliary language). Knowing a language spoken by a large percentage of the population has great benefits. Knowing an auxiliary language spoken by a relative few may be nice but isn't going to offer the same kind of benefits. And if they learned one of the more popular languages, they can use that to communicate instead of an auxiliary language in the game if they choose.
Also, it has been a very long time since I have seen a game that was translated in other languages that was only available in English and Spanish. In most cases, there are at least 5-6 languages to choose from if there is any translation available. Very few go into translations that are not among the most-used languages in the world or the language of the developer. Those that do will still usually pick languages that are among the top 20-30 languages.
You are correct that translations are easy. However, they are time consuming. And if a third party translation service is used, they cost money for each translation. Can you say that a language of convenience that is spoken by relatively few people in the world is worth more than a native language spoken by far more people? I personally would rather see them spend resources on translations for native languages before a language of convenience. There are a LOT of languages in the world. No game will provide translations for all of them. Games that allows mods can have fan translations, which helps. But official translations are going to be limited. They have to pick ones that have the greatest impact rather than ones that might just be nice to have.